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Southwest Wildfires and Global Warming, Explained

The Guadalupe Fire Department fights a wildfire near Nutrioso, Arizona on June 4, 2011. Credit: US Forest Service, Apache-Sitgreaves National Forests.

For much of the Western US, wildfire season hits its peak near the end of summer, after months of hot and dry weather have increased the susceptibility of plants and trees, or “fuels” in wildfire parlance, to burning. But 2011 has already been a banner year for wildfires, in terms of acres burned to date, and the official start of summer is still one week away. At the rate things are going right now, this could be one of the worst wildfire years in recorded history.

As with most extreme weather and climate events, and their related impacts, major wildfires require several factors to come together in order occur — typically some combination of dry and windy weather, abundant and dry vegetation, and a spark, which can range from a carelessly tossed cigarette to a lightning strike.

Wildfires are a naturally occurring phenomenon closely tied to climate conditions, and as the world warms in response to rising amounts of greenhouse gases in the air, many studies show that wildfire frequency and severity will likely shift as well. In fact, retrospective analyses show that the marked increase in recent decades in wildfire activity in the Western US is largely explainable by changes in climate. And climate projections show expected further increases in wildfire extent in parts of the Western US, including some of the areas currently battling massive blazes.

Here are some answers to common questions about this year’s wildfires, including what climate science says about how global warming may change wildfire behavior during the coming decades. This explainer will be updated as the wildfire season progresses and new information becomes available:

How severe has the 2011 wildfire season been so far?

Between January and April of this year, wildfires scorched more acres across the U.S. than during the same period in any other year during this decade. Nearly 1.6 million acres burned in April in Texas alone, the equivalent of the state of Delaware.

According to the National Interagency Fire Center (NIFC), about 4.5 million acres of land have burned in the U.S. so far this year. That’s an area larger than the size of Connecticut. The average amount to date during the period of 2000-2010 is 1.8 million.

Fed by desiccating drought, new wildfires are continuing to ignite across the Southwest, and several have already burned hundreds of thousands of acres each. On June 14, the Wallow Fire raging in eastern Arizona became the state’s largest wildfire on record. As of June 22, it was 58 percent contained. With about 3,500 personnel deployed to combat the flames, firefighters were reporting progress in controlling the blaze.

The hazardous combination of severe drought conditions, gusty winds, and abundant fuels also led to the rapid development of the Track Fire in New Mexico, which quickly spread across state lines into Colorado.

June 17 Update: Another major fire has developed in New Mexico and spread to more than 18,000 acres, near the town of Hereford. The National Weather Service is predicting that this weekend will feature some of the worst fire weather of the season in New Mexico and Arizona, with winds gusting to more than 50 mph, along with very dry air and the chance for dry thunderstorms (storms that don't drop much rain, but whose lightning can spark additional fires). According to the Albuquerque National Weather Service office, the weekend will bring "critical to extreme fire weather conditions", along with "ridiculously windy" weather. Therefore, it's likely that existing wildfires will expand during the next several days, and any new fires may spread rapidly out of control.

June 21 Update: As expected, several new large fires started burning over the weekend in Arizona, New Mexico and Texas, fed by strong winds and dry weather. Critical fire weather conditions exist again today in New Mexico and western Texas, and the outlook for the rest of the week calls for improved conditions there, but critical fire weather conditions may develop in Arizona on Friday.

June 22 Update: 14 new large wildfires were reported yesterday in Texas, and the White Hat Fire grew significantly to cover 42,000 acres, and is only 30 percent contained. For more information about wildfires in Texas, see this interactive map of all current fires in the state. Critical fire weather conditions are still anticipated to develop in northern Arizona on Friday.

June 23 Update: The White Hat Fire grew substantially overnight, now covering over 70,000 acres. A new fire, the Trent Mesa Fire, has ignited in Texas. The fire has covered 500 acres and is already 50 percent contained, but several local wind turbines are caught in the fire's path. Furthermore, critical fire conditions have set in across southwestern Utah, where hot weather, low humidity, and gusting winds around Zion Canyon have increased the potential for new fires to ignite.

Just how large are these fires?

Columbia Helicopters doing bucket work on the Holland-Gage Fire west of Alpine, Texas, on May 8, 2011. Credit: FuelModel3/flickr.

In Texas, wildfires burned more than two million acres of crops, shrubs and grasslands during April and May. Some of the burn scars in Texas are so massive, they are identifiable from space. In total, about two percent of the land area in Texas has burned so far this year. The costs of fighting these blazes already exceed Texas' annual wildfire budget. According to the Amarillo Globe-News, the state has incurred more than $125 million in firefighting costs, compared to the $15.5 million provided for in the state budget.

Arizona’s raging Wallow Fire has already burned about 530,000 acres, more than half the size of Rhode Island.

Why have there been so many large fires in Texas, Arizona and New Mexico this year? What’s causing them?

In Texas, the official fire season started back in November, since much of the Lone Star State experiences its driest conditions prior to the summer. Since that time, a severe drought has tightened its grip; some areas didn’t get any rainfall during March and April, and both of those months were the driest on record for the state.

Similarly, New Mexico recorded its third-driest spring on record. The exceptionally dry weather has led to a very high fire danger. To make the situation worse, weather systems sweeping through the region have tended to bring gusty winds, very low humidity, and only scattered thunderstorms to the region, rather than beneficial rains.

Instead of helping to ease the drought, scattered thunderstorms have ignited several of the wildfires in the Southwest and Texas this year, according to NIFC. But humans are also to blame for a number of the larger fires. The 500 square mile Rock House Fire in Texas was reportedly caused by an electrical short circuit at an abandoned building. Also, the massive Wallow Fire in Arizona is also believed to be human-caused, but this is still under investigation.

However, the fires would not have grown so large (or possibly even ignited in the first place) without the underlying drought conditions.

How have people living in Arizona and Texas been affected?

Property damage has not been as extensive as one might think, given the large size of these fires, but the fires have still been a major problem for tens of thousands of people. For example, while Arizona’s Wallow Fire is still burning out of control, there have only been about 30 homes swallowed by the flames. There were, however more than 2,000 residences and commercial properties considered to be at risk as of June 15.

In comparison, the Rodeo-Chediski fire in June 2002 — previously the largest recorded wildfire in Arizona — destroyed at least 460 homes.

Smoke from the Arizona and New Mexico wildfires has shrouded the skies in much of the Southwest, and affected air quality for people living there. Smoke has also drifted northeast at times, leading to a milky haze in the Midwest and Ohio Valley.

What about climate change? It there any link between global warming and these fires?

Soil moisture index on June 14, 2011, showing very dry conditions in Texas, New Mexico, and parts of Arizona, with very wet conditions across the northern tier of the country. Credit: NOAA/CPC.

Retrospective analyses of historical variations in wildfire activity and climate show that the two are closely linked. Historical variations in climate can explain much of the large year-to-year and decade-to-decade variations in Western US fire activity. Thus, climate change is already increasing wildfire activity in the Western US. This may seem surprising, given the number of other factors (including forest management practices) that are known to affect fire activity.

But climate is a dominant factor because the difference between years with large areas burned and years with small areas burned is not so much in the number of fires, but in the number of very large fires.

When climate conditions are right, a relatively small number of fires grow out of control and consume a lot more acreage.

Interestingly, the link between wildfires and climate change was discussed during a Senate hearing on June 14, according to ClimateWire, when Tom Tidwell, the director of the U.S. Forest Service, told senators that his agency's research shows that climate change is already leading to more frequent and larger wildfires in the West.

"Throughout the country, we're seeing longer fire seasons, and we're seeing snowpacks that, on average, are disappearing a little earlier every spring," he said, noting a trend towards longer fire seasons. "Our scientists believe this is due to a change in climate," he said.

Numerous studies indicate that as the climate warms, fire ecology is likely to undergo potentially significant changes throughout the Western U.S. One study, published in the Journal of Geophysical Research in 2009, found that by midcentury, the average area burned across the entire West is expected to increase by more than 50 percent compared to recent values. The greatest increases are projected to occur in the Pacific Northwest and Rocky Mountains.

Perhaps most worrisome, a recent report from the National Research Council found that 1.8°F of warming from current conditions (arguably the amount of warming to which we are already committed because of our current and past emissions of greenhouse gases) will lead to significant increases in the amount of Western land burned by wildfires, compared to the average area burned during 1950-2003.

In areas that have few plants or trees to burn, the higher temperatures might not have much of an impact, but in parts of Arizona, Colorado, Idaho and Montana, as much as six times more land area could burn each year compared to what is typical today, the NRC panel found. The same effect is likely to unfold for each 1.8°F of additional warming in the future.

Keep in mind, however, that wildfires are naturally occurring, and even necessary for maintaining healthy forests. In fact, there are some plants whose reproductive cycle is triggered by wildfire, making them fire-dependent species. But people have built towns and cities in fire-prone areas across the West, so how climate change influences wildfires is a vital consideration for future land use planning.

I’ve heard that La Niña has been linked to these fires – what is that all about?

Map showing percent of normal precipitation during April 2011. Red shading indicates below average precipitation, while the blue and purple hues indicate wetter than average conditions. Note the very dry conditions in Texas, New Mexico, and parts of Arizona that month. Credit: NOAA.

Generally speaking, years when La Niña conditions are present tend to be wetter in the north and drier in the south (and the opposite for El Niño). La Niña conditions, which are characterized by cooler than average water temperatures in the eastern tropical Pacific Ocean, were present throughout the 2010-11 winter.

La Niña helped steer storms away from the Southwest last winter, leading to the current drought. La Niña typically leads to wet winters in the Pacific Northwest and Ohio Valley, and dry winters in the Southwest. However, this year the typical La Niña pattern (with some variations from the norm) was especially extreme.

Research by Anthony Westerling and Thomas Swetnam shows that La Niña years tend to be the worst for fires in the Southwest, while El Niño years tend to be the biggest fire years for the Pacific Northwest. However, this isn’t always the case.

Westerling and Swetnam have found that a La Niña year preceded by an El Niño event, or a wet winter followed by a dry summer, will result in the most severe fire years in the Southwest and Mountain states of Arizona, Nevada, Utah, Colorado, and Wyoming.

While La Niña very likely contributed to the drought in the Southwest, all extreme weather and climate events now occur in the context of a climate system that is warming due (at least in part) to human activities. As Climate Central’s Heidi Cullen wrote recently: “… All weather takes shape within the broader landscape of climate. Or to use a boxing analogy, climate is the trainer, but weather throws the punches.”